Automatic circuit breaker



Feb. 24, 1959 E. ENDERLI AUTOMATIC CIRCUIT BREAKER Filed June 7, 1955 5 Sheets-Sheet-l Fig.1 5

B/METAL Feb. 24, 1959 E. ENDERLI 2,375,296

AUTOMATIC CIRCUIT BREAKER Filed June 7, 1955 3 Sheets-Sheet 2 m S a F Mw m m Q Q E m u 5 n a QM m u M QR R qfiflsm m ow MW 3E5 #Gzi Emm aw m W Y W W 41.. w Emm mw M F. n A +|Q\ SI x mm W a NM mm 0 v 1| fi Q 9 F m QR.

Feb. 24, 1959 E ENDERLI AUTOMATIC CIRCUIT BREAKER Filed June 7, 1955 s Sheets-Sheet 3 3 I I i I! I! 60 Fig.12 4:

r 4? 3 30033 40 47. 50 A! 32 alldsrzai l 4/a 32 3 BIMETAL 36 36 36.6 35 I; 3 4 2a 40 I BIMETAL 33a 33 WIFB/MEIAL United States PatentO signor to Weber A. G. Fabrik elektrotechnischer Artikel und Apparate, Lucerne, Switzerland Application June 7, 1955, Serial N 0. 513,842 Claims priority, application Switzerland June 8, 1954 -1 Claim. (Cl. 200-116) -The present invention relates to an automatic circuit breaker of the type which serves to protect electrical installations and responds to excessive currents and, more particularly, to an overload of the electrical installation and to short-circuits occurring therein. Automatic circuit breakers of this type are frequently used in lieu of fuses.

It is desirable that such automatic circuit breakers respond to short-circuits instantaneously, i. e. within a period of time in the order of one hundredth of a second, whereas they respond slowly to an excessive load of the electrical installation, i. e. within a period of time of the order fromsome seconds to one hour. Prior automatic circuitbreakers of this type were equipped with electromagnetic means for the instantaneous response to a shortcircuit and with bimetal-controlled means responsive to an overload. The electromagnetic means for instantamenus-response has the additional purpose of quenching the arc incident to a short-circuit. While automatic circuit breakers of this kind afford a protection equivalent to that obtainable by the orthodox fuses, their cost is very high amounting to from fifteen to thirty times the cost of an ordinary safety fuse.

It is the object of the present invention to provide an automatic safety circuit breaker of simple design which responds instantaneously to short-circuits without including any electromagnetic device. More particularly, it is an object of the present invention to provide an automatic circuit breaker in which the separation of the contacts is controlled by a bimetal element, such breaker complying with the ofiicial regulations concerning the responsiveness to overload currents amounting to 1.1 times the normal current and being capable of breaking a short-circuit cur-. rent up to 500 amperes. Prior automatic safety circuit breakers of the bimetal-controlled type which did not contain any electromagnetic devices did not comply with that requirement as they were either too slow in responding to a short-circuit or too sensitive to permit the operation of the installation over a limited period of time at an overload of from 1.1 to 2 times the normal current. It lies within the nature of the fiexure characteristic of bi metal elements heated by the current controlled that the objectionable characteristics of the bimetal breaker cannot be improved by a simple adjustment of the bimetal element, because the correction rendering the breaker sensitive to a short-circuit is inconsistent with the correction that would be required to render the breaker slowly responsive to overload currents. Would it be attempted, for instance, to reduce the time of the response to a shortcircuit by decreasing the fiexure of the bimetal element required to break the circuit, then the period of response to a current amounting to from 1.1 to 2 times the norm would be likewise reduced which is not permissible.

As automatic safty circuit breakers of the bimetal type have been developed which do not contain any magnetic devices for the instantaneous response to short-circuit and for quenching the arc and have the required capacity 2,875,295 Patented Feb. 24, 1959 for breaking powerful currents, it is a more specific object of the present invention to so modify such prior bimetal circuit breakers that the prescribed response periods will be attained over the entire range of currents, thus ensuring that when a short-circuit occurs the contacts will open within a period of time that does not exceed the prescribed maximum and is as short as possible, such opening of the contacts being caused by as small a flexure of the bimetal element as possible, additional means being provided to artificially extend the period of response to an overload current amounting to from 1.1 to 2 times the norm.

Further objects of the present invention will appear from a detailed description of a number of embodiments thereof following hereinafter with reference to the draw: ings. It is to be understood, however, that such detailed description serves the purpose of illustrating the invention rather than that of restricting or limiting the same. In the drawings,

Fig. l is an elevation of an automatic circuit breaker in closed condition,

Fig. 2 is a side view of Fig. 1,

Fig. 3 illustrates the bimetal element of the circuit breaker shown in Figs. 1 and 2,

Fig. 4 is an elevation of a circuit breaker similar to that of Fig. 1,

Fig. 5 is a side view thereof,

Fig. 6 is an elevation of another circuit breaker embodying the present invention and slightly differing from that shown in Fig. 4,

Fig. 7 is a side view of the breaker shown in Fig. 6,

Fig. 8 is an axial section of an automatic circuit breaker adapted to be screwed into a fuse socket, the section being taken along the line 88 of Fig. 10 and showing the contacts in closed condition,

Fig. 9 is a sectional view of the same circuit breaker, the housing being shown in section taken along the line 9-9 of Fig. 10,

Fig. 10 is a cross section taken along the line 10-10 of Figs. 8 and 9,

Fig. 11 is a partial sectional view similar to that of Fig. 8 showing the contacts in separated condition, and

Fig. 12 is a sectional view similar-to that of Fig. 17, but showing the contacts in separated condition.

In Figs. 1 to 3, 1 denotes a bimetal element through which the current flows, 2 are movable contacts mounted thereon, 3 denotes stationary contacts carried by. terminal members 4 provided with clamping screws 5. 7 denotes heat-dissipating fins of metal or ceramic material of a high heat conductivity. Such fins may be formed of bimetal as shown. 8 denotes means for opening the con-. tacts, and more particularly springs acting on the bimetal element carrying the contacts. 9 denotes a stationary stop adapted to be engaged by the bimetal element.

Since these illustrations are more or less diagrammatic and do not show the means for relatively guiding the contacts, they will be more easily understood after the embodiment of the present invention illustrated in Figs. 8 to 12 will have been described in detail.

In these figures an automatic circuit safety breaker is shown which is adapted to be screwed into a socket of a fuse as a substitute therefor. The breaker has a plugshaped housing of insulating material which is divided lengthwise into sections 31a and 31b and has external threads adapted to receive a threaded sheet metal ring 44 which holds the sections 31a and 31b together and'which constitutes an Edison thread permitting the housing to be screwed into a fuse socket. Moreover, the housing is provided with a bore in its top and with a cap 49 of insulating material fitted on the top portion of the housing 7 and provided with a central bore and an internal collar 50 surrounding such bore and extending into the bore provided in the top of the housing.

The bottom of the housing is provided with a bore accommodating a flanged metal plug 45 which. .has .a threaded end portion projecting from the housing 31a, 31b and engaging internal threads of .a bottom contact 43. In the embodiment shown, this bottom contact is cylindrical and the lower portion thereof has external threads.

A re-setting member of insulating material in :form ,of a bolt provided with a peripheral shoulder 35 between its ends has an upper cylindrical portion 32 slidably guided in the cylindrical bore of the cover 49 and projecting therefrom to form an actuating button, and a reduced lower end 34 slidably guided in an internal. bore of the member .45. The cylindrical portion 33a from which the reduced portion 34 extends downwardly is fitted in a bracket 33 and rests on an internal flange of such bracket. The latter is urged in upward direction by'a helical spring 46 which surrounds the reduced portion34 of the "resetting member and is inserted between the bracket 33 and the member 45. As long as the contacts are closed, the re-setting member is arrested in the position shown in Figs. 8 and 9 contrary to the tendency of spring 45 by engagement of the collar 35 with the bimetal'element to be described later. When the bimetal element responds either to a short-circuit current or to overload, it will undergo a deformation'causing it to release the collar 35 permitting the spring 46 to move the re-setting member upwardly .into the position shown in Figs. 19 and 20 in which a collar 32b of the resetting member engages the collar 50 of the cap 49. r

A flanged sleeve 51 of resilientmaterial surrounds the re-setting member and, more particularly, the collar 32b thereof and has its flange clamped between the housing and its cap. When the gaseous medium in the housing is subjected. to pressure by the heat developed by arcs produced incidentally to the breaking of a shortcircuit current, the compressed gaseous medium presses the sleeve 51 againstthe collar 32b to thereby seal the gap between the knob 32 and the cap 49 of the housing.

This is desirable since high pressure of the gaseous rrledium will assist in quenching the arc. The sleeve 51 preferably consists of a material capable of resisting high temperatures.

Moreover, a sheet metal cap 48 may be inserted between the cap 49 and the housing to assist in holding the housing sections 31a and 31b together against high internal pressure.

The external thread of bottom contact 43 is selected in dependence on the nominal current of the automatic safety breaker. Preferably, a diameter of 6 millimeters is employed for 6 amperes, a diameter of 8 millimeters for 10 amperes, a diameter of 10 millimeters for amperes, a diameter of 12 millimeters for amperes, and a diameter of 14 millimeters for amperes. Where the safety breaker is to be inserted into the socket of an ordinary screw fuse by means of the Edison thread 44, the threads of the bottom contact 43 will just pass through the corresponding holes of the sockets. However, special sockets may be provided having a threaded hole into which the bottom contact 43 may be screwed. In this event, the special socket will not be provided with threads for engagement by the Edison threads 44, but will be provided with an annular contact shoulder adapted to be engaged by the shoulder provided at the bottom of threads 44. If desired, however, the bottom contact may have a standard thread of 6 millimeters diameter, for instance, for all nominal currents and may have a reduced end, the diameter of which is dimensioned to suit the nominal current. Alternatively, the bottom contact 43 may be free from any threads and itsdiameter may be dimensioned in dependence on the nominal current, as is customary with screw fuses.

The portions 32, 32a, 33a, 34 and 35 may be formed 4 by individual elements which are rigidly connected with one another by a suitable adhesive.

It will be noted from Figs. 8, 9, l1 and 12 that an enlarged chamber is provided in the upper portion of the insulating housing. .A pair of stationary contacts 42 is mounted on the ceiling of the enlarged chamber on either side of the sleeve 51 in diametrical opposite relationship. One of these contacts is carried by a conductive strip 41a extending along the ceiling of the chamber outwardly into the wall of the insulating housing being embedded therein and extending downwardly into contact with the bottom contact 43, as will appear from Figs. 8 and 9. The other contact 42 is likewise carried by a conductive strip 41b which extends along the ceiling of the chamber in outward direction and into the wall of the housing being embedded therein and leading downwardly into contact with the threaded sheet metal ring 44. Vertical ribs 52 are provided on the internal wall of the housing for the purpose of extending the creeping current path between the contacts 42.

A pair of movable contacts 37 disposed below the pair of contacts 42 is mounted on a bimetal element 36. The bimetal element includes bridge portions 62 and a pair of U-shaped portions 36a and 3612.. Each of these portions 36a and 36b is a vertical flat U-shaped plate of bimetal, the arms of which extend upwardly from a base 61 illustrated in Fig. 17 in section. The opposed,upwardly extending arms of the .U-shaped portions are integral with the horizontal bridge portions 62 extending on either side of the re-setting member transversely to the same. As will appear from the drawings showing member 36 under normal temperature conditions, the two flat U-shaped plates are thus held in vertical spaced parallel relationship, their distance exceeding the diameter of the portion 32a of the re-setting member and of the collar 35 thereof. Thus, the re-setting member ex tends between the two U-shaped portions 360 and 36b and also between the two bridge portions 62. Suitable mounting means are provided for guiding the bimetal element 36 for movement towards and away from the pair of'staticnary contacts 42. In the embodiment shown such mounting means comprises a pair of protective burnoff sheet metal members 38 which are placed on the bridge portion 62 of the bimetal element 36 and are suitably secured thereto, for instance by rivets which serve at the same time to secure the movable contacts 37 in position. Each of the sheet metal members has a pair of projections 39 (Fig. 18) which embraces a square section of portion 32a of the re-setting member and is slidably guided on the edges thereof for vertical movement.

When the contacts are opened, arcs burning between the two pairs of contacts will jump over from the contacts 42 to the sheet metal members 38 and the same are made of a material which is not liable to burn off excessively.

For the purposes of the present invention each of the U-shaped portions 36a and 35b is provided with a fin of a heat-conductive material which extends from the bimetal portion and will dissipate the heat produced therein into the surrounding atmosphere. Each of the two fins is formed by a U-shaped bimetal strip 40a, or 4311 respectively, which is disposed in upright inverted position and has its outer leg fixed, for instance welded, to the base 61 of the U-shaped portion and has the other leg extending between the base 61 and the re-setting member into contact with the upper shoulder formed by the collar 35 thereof, as shown in Fig. 9.

A pair of springs 47 constituting means for opening the contacts tends to pull the contact-carrying bimetal member 36 downwardly relative to the re-setting member but is normally prevented from so doing by engagement of the fins Mia, 40b with the collar 35. .Each spring 47 is a helical spring having one end anchored to the bracket 33 secured to the re-setting member and having the other end secured to a lip 63 which is punched out of the sheet metal member 38 and bent downwardly therefrom.

The bimetal element 36 consists in a known manner of two layers of metal which are welded to each other and have different coeflicients of heat expansion. As the current is conducted from the bottom contact 43 through the conductive strip 41a, the stationary contact 42, the movable contact 37 in engagement therewith, the associated bridge portion 62, the pair of U-shaped portions 36a and 36b, the other movable contact 37, the associated stationary contact 42, and the conductive strip 41b to the threaded sheet metal ring 44, the U-shaped portions 36a and 36b will be heated when the current exceeds its norm and by such heat are subjected to a deformation causing the U-shaped portions to be deflected in the direction of the arrows 36a and 36b in Fig. 17 outwardly. When such deflection exceeds a certain limit, it will cause disengagement of the portions 40a and 40b of the bimetal element 36 from the collar 35 of the re-setting member thereby permitting the springs 47 to pull the bimetal element 36 downwardly into the position shown in Figs. 11 and 12 in which the pairs of contacts are separated. Moreover, it will be noted that the enlarged chamber of the insulating housing forms a pair of internal shoulders 60 for engagement by the rivet heads of the movable contacts 37.

As the U-shaped fin portions 40a and 40b likewise consist of bimetal, they will be likewise subjected to heat flexure. However, their limbs are spread apart by said heat flexure. Hence, the heat flexure occurs in the direction indicated by arrows 40a and 40b opposite to the deformation 36a and 36b of the U-shaped portions 36a and 36b. Consequently, the heat flexure of the fin portions 40a and 401: will partly compensate the deformation of the U-shaped portions 36a and 3612.

If desired, the member 45 and the bottom contact 43 may be made integral with each other.

The displacement from normal position of the lower ends of the bimetal element 36 which in the embodiment shown are formed by the inner limbs of the U-shaped fins 40a and 40b, may amount to 0.3 millimeter, for instance. Upon such displacement the bimetal latching mechanism formed by the bimetal element 36 and the re-setting member'provided with collar 35 will be unlocked and will release the contacts 37 for separation by the springs 47.

When the electrical heating of the U-shaped main bimetal portions 36a and 36b constituting the path for the current occurs slowly by a slight overload, the current exceeding its norm from to 20 percent, the heat produced in the main portions 36a and 36b will be transferred therefrom to the auxiliary U-shaped bimetal portions formed by the fins 40a and 40b which are located out of the path of the current. These auxiliary portions, however, are flexed in the direction of the arrows 40a and 40b thereby keeping in engagement with the collar 35 even though the temperature of the main bimetal portions 36a and 36b may reach a rather high limit. Not until the current exceeds its norm by more than for instance 20 percent will the deformation in the direction 36a and 36b exceed the flexure in the direction 40a and 40b by more than 0.3 millimeter thus causing disengagement.

Hence it will appear that the fins 40a and 40b render the safety breaker comparatively insensitive, as far as an overload of the installation is concerned. These fins, however, do in no way affect the responsiveness of the novel automatic circuit breaker to short-circuits. When a short-circuit occurs, the main U-shaped portions 36a and 36b are so suddenly heated that their deformation .36!) reaches a limit far lower than the temperature limit 6 that causes the breaking of the circuit in response to an overload. In other words, the provision of the fins 40a and 40b does not affect the response of the novel circuit breaker to a short-circuit.

For the purpose of re-setting the breaker to contact closing condition in which the elements assume the posi tions illustrated in Figs. 8 and 9, the button 32 must be depressed causing the collar 35 to descend below the lower ends of the inner limbs of the auxiliary U-shaped bimetal fins 40a and 40b at the same time depressing bracket 33 thereby compressing spring 46 and stretching the springs 47. On subsequent release of the button 32 the spring 16 will lift the re-setting member, and the collar 35 thereof will rise into engagement with the bimetal element 36 resting on the internal housing shoulders 60 On further upward movement of the re-setting member, the collar 35 thereof will lift the bimetal element from the shoulders 60 and will return the elements into contact-closing positions shown in Figs. 8 and 9.

From the above description it will be evident that in the embodiment shown in Figs. 8-12 the bimetal element 36 constitutes in effect a latch which consists of bimetal and in normal condition is operative to lock the contacts in closed relationship. When abnormally heated, the latch releases the contacts for separation by the springs 47 Hence it is apparent that the fins 40a and 40b forming part of the bimetal element 36 are in effect delaying means which are responsive to a slow heat production extending over a period of time exceeding a few seconds and delay the effect of the deformation of the main bimetal portions 36a and 36b, but which do not respond to a quick heat production caused by a short-circuit whereby the novel safety breaker will slowly respond to an overload but will quickly respond to a short-circuit. Thus, the breaker is characterized by the provision of the bimetal element with means influencing the deformation of the bimetal element when the same responds slowly over a period of time from one hour to a few seconds, such means being not effective when the response occurs within a brief period in the order of a few seconds or less.

Therefore, the novel safety breaker complies with the requirement that when a short-circuit occurs, the opening of the contacts must be effected within a limited period of time as quickly as possible. In order to comply with this requirement, the amount of displacement of the latching means resulting in a release thereof from the stop or collar 35 has been reduced to a very short distance and, therefore, the responsiveness to an overload current exceeding the norm by 10 to 20 percent has been artificially delayed by the provision of the delaying means, such as 40a and 40b.

The artificial delay of the response to a current exceeding the norm by 10 to 20 percent is partly due to the fact that the heat produced by the current in the main bimetal portions, such as 36a and 36b, is transferred to the fins, such as 40a and 40b, and dissipated thereby into the surrounding atmosphere. Owing to such heat dissipation the main bimetal portions are effectively cooled. As a result, it takes a longer period of time until the novel breaker will respond to the overload current and break the same. When the overload amounts to from 10 to 20 percent of the norm, the contacts will be opened after a period of time which may extend from thirty seconds to thirty minutes. Within such periods of time the heat produced in the main bimetal portion will be partly transferred to its fin and dissipated thereby to the surrounding atmosphere, thus attaining the object of the present invention. Such transfer, however, will not take place to any substantial extent when the heat is produced within a fraction of a second as a result of a short-circuit occurring in the installation protected by the novel automatic circuit breaker. Therefore, the latter responds to a shortcircuit within'.01 to .5 second.

with the standard regulations.

In the embodiment described *the retarding efiect produced by the fins 46a and 40b is enhanced by'the fact that such fins themselves consist of bimetal and are thus subject to a heat fiexure partly compensating that of the main bimetal portions. That is the reason why in the embodiment shown the fins rather than the main portions 7 of the bimetal elements cooperate with the stop 35.

If the fins 40a and.40b were omitted and if the base portions 61 of the bimetal element would be so disposed as to engage and cooperate with the stop 35, the dimensions of the elements being otherwise the same, the breaker would respond within about three minutes to an overload current exceeding the norm by 20 percent. The threeminute period, however, is much too short to comply If the fins would consist of ordinary metal as distinguished from bimetal, the dimensions being otherwise the same, the breaker would respond to an overload current exceeding the norm by 20 percent after a period of time in the order of fifteen minutes. Owing to the fact that the fins consist of bimetal, the period of response to an overload current exceeding the norm by 20 percent has been extended to a period in the order of twenty-five minutes, for instance. Any desired extension of such period, that is to say any desired delay of a response to an overload current, may be attained by suitably dimensioning the fins.4tla and ibb without adversely afiecting the responsiveness of the breaker to a short-circuit.

In the embodiment shown, the bimetal element itself constitutes in effect a latch locking the contacts in closed condition and releasing the same for spring-actuation for the purpose of breaking the contacts. Alternatively, however, a separate latch may be provided in addition to the bimetal element and may be cooperatively connected therewith for engagement and disengagement.

Preferably, the bimetal element conducting the current to be controlled is of U-shapeand cooperates with the resetting member. This arrangement offers particular advantages where the invention is applied to a safety switch of the type adapted to be substituted for a screw. fuse. An automatic circuit breaker of this type offers the advantage that it affords the possibility of closing the contacts by simple manipulation after the cause of the overload or the short-circuit has been eliminated as distinguished from a screw fuse which must be replaced in event of a response to an overload or ashort-circnit.

Moreover, it will appear that the novel automatic circuit breaker consists of simple elements lending themselves to cheap manufacture and assembly as distinguished from prior circuit breakers including electromagnetic devices for the purpose of quickly responding to shortcircuits.

The novel automatic safety switch serving to protect electrical installations from excessive overload and from damage by short-circuits is provided with a re-setting button cooperating with the bimetal element, the latter including the described delaying means which delay the response of the safety, switch to an overload but will not delay the response to a short-circuit.

On the basis of a full understanding of the invention conveyed by the above detailed description of the safety switch illustrated in Figs. 8 to 12, the diagrammatic illustrations in Figs. 1 to 7 will be readily understood.

In the embodiment illustrated in Figs. 1 to 3, it is the fin 7 of the bimetal element 1 rather than the main portion thereof that cooperates with the stationary stop 9. in this embodiment the fin 7 consists of bimetal.

The embodiment illustrated in Figs. 4 and 5 has a shape of the bimetal fin 26. The stationary contacts 23 are mounted on terminal members 24 provided with clamping screws25. The U-shaped bimetal element 21 carries the movable contacts 22 and is guided for up and down motion by suitable means, not shown. The bimetal fin 26 is U-shaped and has one ofits'limbs suitably secured,

for instancewelded, to the base portion of the bimetal member 21 overlying such base portion. Springs '28 tend to pull the bimetal element 21 downwardly to thereby open the contacts. I

The embodiment shown in Figs. 6 and 7 difiers from that shown in Figs. 4 and 5 in that the U-shaped bimetal fin 27 is integral with the base portion of the bimetal element 21, the entire bimetal element including the fin 27 being produced by a punching and bending operation. Hence, welding of the fin portion to the main portion of the bimetal element is eliminated. This embodiment ofiers the advantage that the U-shaped bimetal element will act both as a limit current compensator and as a compensator for the temperature of the atmosphere. This embodiment also permits a quicker re-setting after having been released in response to an overload or a shortcircuit. 7

While the invention has been described in connection with several different embodiments thereof, it will be understood that it is capable of further modification, and this application is intended toscover any variations, uses, or adaptations ofthe invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains, and as fall within the scope of the invention or the limits ofthe appended claim.

What I claim is:

Automatic circuit breaker responsive to overload and to 'short-circuiting comprising a pair of contacts,'means for opening said contacts, a bimetal element in series with said contacts to be heated by the current under control and adapted to actuate said opening means by heat for movement towards and away from said pair of stationary contacts, said bimetal element including a pair of U-shaped portions, a pair of movable contacts each mountedon said U-shaped portions in position for en- 'gagement and disengagement of said pair of stationary contacts, resilient means mounted to act on said bimetal element and tending to move same into a contact-separating direction,and a stop disposed between said U-shaped portions, said bimetal members being shaped and disposed to normally engage said stop to be locked incontact-closing position contrary to the tendency of said resilient means, but being adapted by heat flexure to disengage said stop and to thereby cause said resilient means to become effective and to move said bimetal element to contact-separating position, said U-shaped portions of said bimetal element constituting a current path between said pair of movable contacts, said bimetal members disposed for heat fiexure in a direction oppo-siteto the heat deformation of said U-shaped portions being thereby adapted to partly compensate said deformation in said U-shaped portions, said circuit breaker further comprising a housing of insulating material encasing said contacts, said bimetal element, said resilient means and said stop, and a re-setting member slidably guided in said casing for movement in the direction of said first mentioned movement and forming said stop, saidre-setting element being formed with a button protrudingfrom said housing, the latter being formed with external threads for insertion in a fuse-socket and with a bottom contact electrically connected in series with said afore-mentioned contacts, said circuit breaker, further comprisinga sleeve of resilient material surrounding said re-setting member gaseous medium insaid housing is subjected to pressure,

to engage said button to thereby seal the gap between said button and said housing.

References Cited in the file of this patent UNITED STATES PATENTS Esser Nov. 17, 1931 Sachs June 18, 1935 Dorfman Aug. 11, 1936 Werner Sept. 28, 1937 10 Hays et a1. Mar. 16, 1943 10 Walker et a]. Sept. 18, 1951 Ingwersen Feb. 26, 1952 Ingwersen Jan. 13, 1953 Ingwersen May 19, 1953 Ingwersen Jan. 5, 1954 Jackson et a1. Dec. 14, 1954 FOREIGN PATENTS Great Britain Apr. 25, 1935 Great Britain Apr. 25, 1935 Great Britain May 31, 1937 

